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Hydrogen breath test

This page outlines what to expect both during and after the test. You will need to do some test preparation, so please read everything carefully.

Hydrogen breath tests 

A hydrogen breath test is a safe, non-invasive test used to investigate certain intestinal disorders. Your consultant will have referred you to assess whether you have one or both of the following:

  • Small bowel bacterial overgrowth: This is a condition where there are abnormal numbers of bacteria in the small intestine.

    or

  • An intolerance to a specific sugar: An intolerance is when the body is unable to sufficiently break down a specific food (e.g. lactose which is the sugar found in dairy products).

Please be aware that if you have been referred for more than one breath test, these cannot be performed on the same day.

Upon arrival 

You will be greeted and taken into a private room by a member of the GI physiology team who will be performing the test. They will ask you some questions about the symptoms you have been having and take a brief history. They will also take this opportunity to answer any questions you have or any concerns regarding the test.

How shall I prepare for the test?

  • You need to fast for at least 14 hours before the test and not drink anything apart from water (particularly avoid milk and fruit juice the day before the test). If you are diabetic, please see the frequently asked questions below.
  • Avoid high fibre foods the day before the test including: fruit, vegetables, wholegrains.
  • Try to base meals around white bread, plain white rice/ pasta, potatoes (skin off), chicken or fish. There is a suggested meal plan below.
  • Your last meal on the day before the test should not be too big and should ideally not consist of any roughage (e.g. beans, cabbage or leeks).
  • You must not smoke/vape or chew gum 12 hours before the start of the test.
  • Avoid laxatives for 3 days before the test (especially lactulose).
  • Medicines (apart from vitamins, laxatives and antibiotics) can be taken with plain water on the day of the examination.
  • If you use dentures, do not use an adhesive on the day of test.
  • Brush your teeth on the day of the test.

The following will make the test results difficult to interpret:

  • Antibiotics in the last 4 weeks.
  • Colonoscopy in last 4 weeks.
  • Irrigoscopy in last 4 weeks.
  • Other bowel cleansing procedures in last 4 weeks.
  • Ileostomy (except for diagnosis of bacterial overgrowth).

Please contact the department if any of the above is unclear or you have had any in the last month (see below).

What does the test involve?

  • You will be asked to give an initial breath sample into a hand-held machine. This is done by holding your breath for 15 seconds and then blowing gently into a mouthpiece at a steady rate.
  • A cup of sugar dissolved in water will be then given to you to drink.
  • Additional breath samples will be taken every 20 minutes for 2-3 hours, while the test solution travels through the gastrointestinal tract.
  • During the test you should not eat, chew gum, smoke, sleep, or exercise.
  • You can continue normal activities once the test is completed.

Repeat testing 

If your initial breath sample is not sufficiently low enough (e.g. due to poor test preparation), we may have to re-book the test for another day. If this happens, we will ask you to come back with the following instructions:

  • Fast for at least 16 hours before the test.
  • Don’t eat any roughage (fibrous foods like beans, cabbage, and leeks) before the test.
  • If lactose intolerance is a possibility the last meal consumed prior to the test should not contain milk or dairy products. If fructose intolerance is suspected the last meal should not contain fruit.
  • On the morning of the test drink a glass of warm water (200-300ml).

Frequently asked questions

Are there any risks associated with this test?

Hydrogen breath tests are very safe. In some cases the test my trigger your usual symptoms. 

The hydrogen breath test is only dangerous in the following two (rare) scenarios: 

  • If hereditary fructose intolerance is suspected (or known) you must not undertake the fructose load test or sorbitol load test.
  • If you have postprandial hypoglycaemia (low blood sugars after eating) of unknown cause, you must not have a hydrogen breath test.

What will happen after your test?

A report with your results will be sent to your referring consultant or GP. If you require treatment, your doctor will arrange this. You may be asked to return for another appointment if an additional test using a different sugar has been asked for. You will be able to resume normal activities following the test.

Are there any alternatives to the test?

There are no alternative tests for diagnosis other than a trial of antibiotics in the case of suspected small bowel bacterial overgrowth, or dietary exclusion in the case of suspected sugar intolerances.

How long does the test take?

The appointment is for 2-3 hours with breath samples being taken every 20 minutes. This can feel timely and therefore please feel free to bring a book or magazine.

Diabetic patients

If you are diabetic please let our department know using the contact information below. 

Please bring the following to the appointment:

  • Your blood glucose monitor (if you have one).
  • Food to eat after the test.
  • Insulin (if you use this).

Further appointments 

If you require any additional information concerning the investigations or any advice please contact us using the details below: 

  • If unavailable please leave a message and you will be contacted.

Suggested meal plan 

A bland diet the day before the test is required to avoid compromising the results of the test. Please avoid roughage 

  • Breakfast: Plain white toast. Poached or hard boiled eggs.
  • Lunch: White bread sandwich with chicken/ham/tuna/tofu, a plain egg omelette (no butter or milk), or tofu scramble.
  • Dinner: White rice, chicken (no breadcrumb coating), seafood, pork, baked white potato (no skin).
  • Drinks: Water, black tea/ coffee (no milk or sugar).
  • Snacks/alternative: Small amounts of peanut butter.

References and sources of additional information

The Bladder and Bowel Foundation 

The Bladder and Bowel Foundation is a UK wide charity dedicated to helping people manage their continence needs as a result of both bladder and bowel control problems. They can be contacted at: 

British Nutrition Foundation 

This organisation is a registered charity that provides information on food and nutrition. They can be contacted at: 

Allergy UK 

This organisation is a national charity that provides information and advice on allergies and intolerances. They can be contacted at: 

© North Bristol NHS Trust. This edition published May 2024 Review due May 2027. NBT003005.

Contact GI Physiology

Gate 36, Level 1,
Brunel building, 
Southmead Hospital,
Bristol
BS10 5NB

Phone: 0117 414 8801
Email: GIphysiology@nbt.nhs.uk

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Hydrogen breath test